Tests For Positive ions

Flame Test:

  • Put a small amount of the compound to be tested in a nichrome loop.
  • Then hold the loop in a blue flame of a bunsen burner.
  • Then use the colour of the flame to identify the metal ion.

Lithium (Li+) = Crimson

Sodium (Na+) = Yellow

Potassium (K+) = Lilac

Calcium (Ca2+) = Red

Barium (Ba2+) = Green

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  • Created by: Chiara
  • Created on: 04-04-13 10:40

Flame Test

Flame Test (Positive ions)

  • Put a small amount of the compound to be tested in a nichrome loop.
  • Then hold the loop in a blue flame of a bunsen burner.
  • Then use the colour of the flame to identify the metal ion.

Lithium (Li+) = Crimson

Sodium (Na+) = Yellow

Potassium (K+) = Lilac

Calcium (Ca2+) = Red

Barium (Ba2+) = Green

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Reactions with Sodium Hydroxide

Sodium Hydroxide

  • Aluminium, Calcium and Magnesium ions form white precipates, so when we react it with sodium hydroxide we know that the unknown compound cantains either; Al3+, Ca2+ or Mg2+ ions.
  • If we add more sodium hydroxide then the white precipitate formed with aluminium ions dissolve.
  • Calcium and Magnesium ions can be distinguished by a flame test. Magnesium ions produce no colour.

We can identify ions in a substances by following this:

Copper ions = Blue precipitate

Iron ions (l) = Green precipitate 

Iron ions (ll) = Brown precipitate

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Carbonates

Carbonates (negative ions)

  • If we add dilute acid to a cabonate it fizzes and produce carbon dioxide.
  • This is a good test to see if an unknon substance is a carbonate.
  • In limewater the carbon dioxide reacts with calcium hyroxide. It forms a white precipitate of calcium carbonate which turns the limewater cloudy.
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Halides

Halides (chloride, bromide and iodide)

  • The test shows whether chloride, bromide or iodide are present in the compound.
  • First we add dilute nitric acid, then add silver nirtrate solution, this removes any carbonate ions.
  • If a precipate forms then a halide is present in the compound.

To indentify what ion is present we need to look at the colour of the precipitate:

Chloride ions = White precipitate

Bromide ions = Cream precipitate

Iodide ions = Pale yellow precipitate

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Sulfates

Sulfates (negative ions)

  • We can test for sulfate ions by adding dilute hydrochloric acid, followed by barium chloride solution.
  • The dilute hydrochloric acid removes carbonate ions .
  • A white precipitate tells us sulfate ions are present
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